Polymer material for medical instruments and method for production of the polymer material

ABSTRACT

A polymer material (6) is formed from a first polyethylene component (1) having a low density (VLDPE) which is silane-grafted with an organic silane (3) in combination with organic peroxide (4) and is cross-linked through storage in a damp environment and/or in water. The polymer material (6) can be processed into medical instruments under dry conditions prior to cross-linkage. After cross-linkage the medical instruments made from the polymer material (6) exhibit a high degree of transparency, are non-buckling and flexible. The polymer material (6) is produced in an extruder, preferentially in a double-worm extruder.

BACKGROUND OF THE INVENTION

The invention concerns a medical instrument comprising at least oneshaft section having a lumen, wherein the shaft section or sections aremade from a flexible polymer material.

The invention also concerns a method for the production of a polymermaterial.

These types of medical instruments, for example, catheters, tubes,tracheal tubes, and the like are, as is known in the art, produced froma plurality of polymer materials. Towards this end, thermoplastics aswell as elastomers are utilized. Among the range of thermoplastics, softPVC materials are still normally used. Due to the low softeningtemperature of soft PVC materials, sterilization of medical instrumentsmade from this material using hot steam is not possible. Conventionalmedical instruments made from soft PVC materials can therefore only beused one time in a sterile condition (disposable instruments). As aresult, utilization of these medical instruments generates a largenumber of contaminated instruments which must be disposed of. Onedisposal possibility is incineration of the contaminated PVC material.Since dioxin is thereby produced, this type of disposal is controversialeven when subjected to the most stringent safety conditions.

The PVC polymers which are utilized for medical instruments usuallycontain softeners for achieving the desired instrument flexibility. Thesofteners are usually simply physically mixed into the PVC polymer sothat it is possible for these materials to diffuse out of the PVCmaterial and into its immediate surroundings. In the event that medicalinstruments of this type are placed inside humans, it is possible forthe softener to enter into the body.

Very-low-density polyethylene (VLDPE) and ultra-low-density polyethylene(ULDPE) are softener-free, soft polymers with characteristic featurescomparable to soft PVC material and have, as do the soft PVC materials,very low softening temperatures and are therefore not suitable for steamsterilization.

In addition to thermoplastic components such as the kind utilized inmedicine, medical instruments are also made from rubber materials whichhave the advantage, due to their chemical cross-linked structure, ofbeing steam-sterilizable. Rubber materials are non-transparent, requirecostly processing technology and, compared to thermoplastics, arerelatively expensive raw materials.

It is therefore the purpose of the present invention to develop apolymer material for medical instruments which is physiologicallyunobjectionable, transparent, which can be sterilized by steam, andwhich has the necessary flexibility while maintaining a constantly openlumen.

It is furthermore the purpose of the present invention to present amethod for the production of a polymer material of this type.

SUMMARY OF THE INVENTION

The purpose of the invention is achieved with respect to development ofthe polymer material in that the polymer material is formed from a firstpolyethylene component having low density (VLDPE) and/or a secondpolyethylene component having extreme low density (ULDPE), whereby anorganic silane is grafted to the polymer material with the addition ofan organic peroxide and the grafted polymer is cross-linked throughstorage in a humid environment and/or in water.

The above mentioned purpose in accordance with the invention is solvedwith respect to a method for production of a polymer material of thistype in that the first polyethylene component and/or the secondpolyethylene component are dosed by weight and introduced as pourablebulk material to an extruder, preferably a double-worm extruder, and amixture comprising organic silane and organic peroxide and, ifappropriate, a catalyst is injected into the extruder, preferably usinga membrane dosaging pump having a cooled injection valve, and a vacuumfor degasing the melt is applied to the extruder in the vicinity of theproduct discharge with the extruder being heated at least between theregion at which the organic silane and the organic peroxide isintroduced up to the product discharge and medical instruments areproduced from the degased melt of silane-grafted polymer material underdry conditions and subsequently cross-linked by exposure to moisture.

The polymer material in accordance with the invention has the advantagethat it has an increased resistance to buckling compared to the initialpolymer material. An undesired narrowing of the cross section of thelumen in the event of bending of the medical instrument is opposed bythe polymer material itself. In addition, the polymer material inaccordance with the invention is free of softeners, surprisinglyexhibits the desired transparency and can be steam-sterilized aplurality of times at the medically required temperature of T=134° C.The polymer material in accordance with the invention uses a rawmaterial which is more economical than comparable rubber products andthe processing of polymer material in accordance with the invention intomedical instruments is possible in a conventional manner under dryconditions.

The processing of the polymer material in accordance with the inventioninto medical instruments can take place directly after production of thesilane-grafted polyethylene melt. The silane-grafting and the extrusioninto a tube can also be carried out with an extruder in a single step.The polymer material in accordance with the invention can, however, alsobe further processed after cooling or after further granulation. In theevent that the polymer material in accordance with the invention isstored in a moisture-proof fashion in suitable packaging (for examplewelded plastic bags), it can still easily be further processed followingstorage times of several weeks.

The method in accordance with the invention facilitates the productionof the silane-grafted polymer melt in an economical and reproduciblefashion using simple processing steps. The reacting chemicals arehomogeneously distributed in the first and/or second polyethylenecomponents using conventional and reliable machine technology so thatone obtains an end product which is free of gel particles and pinholes.

If, in a preferred embodiment of the invention, a catalyst or a catalystmixture is added to the polymer material the cross-linking time can beprecisely determined. The processing time is likewise shortened, forexample the time for the welding of components. Dibutyltindilaurate(DBTL) and/or a titanylacetonate are particularly well suited ascatalysts. A mixture of a plurality of suitable chemicals can also beutilized as a catalyst.

In a further embodiment of the invention, the first polyethylenecomponent and/or the second polyethylene component have a narrowmolecular mass distribution. The molar mass distribution lies in therange of 1.5≦M_(w) /M_(n) ≦3.0. Thereby M_(w) respresents theweight-averaged molecular weight and M_(n) the number-averaged molecularweight of the polyethylene components. The ratio of M_(w) to M_(n) asformulated in patent claim 4 characterizes the width of the molecularweight distribution. It is preferred in accordance with the invention tochoose a polyethylene component or polyethylene component mixture havinga ratio M_(w) /M_(n) of 2. This type of molar mass distributionfacilitates a surprisingly high gel-content in the vicinity of 65%≦C_(G)≦95% which cannot be achieved with conventional VLDPE and also a highdegree of cross-linking in the end product, a high degree ofcross-linking being a requirement for steam-sterilizability.

A further embodiment of the invention utilizes an organic peroxide whosedecomposition temperature region lies above the melt temperature of thefirst polyethylene component and/or of the second polyethylenecomponent. Dicumylperoxide (DCUP), Dibenzoylperoxide (DB) and/ordimethylhexanebutylperoxide (DHBP) are preferentially utilized asorganic peroxides. The organic silane utilized is added to thepolyethylene component or components in weight ranges of 0.5 to 5% byweight and the organic peroxide in the range between 0.02 to 0.3% byweight.

Dibutyltindilaurate (DBTL) in the range from 0 to 0.05% by weight ortitanylacetonate in the range from 0 to 0.5% by weight is added ascatalyst to the polymer material in accordance with the invention.

A functional polymer material can be obtained from a flexiblepolyethylene material of low or extremely low density using the smallestfractions of reaction chemicals and can be processed under dryconditions into medical components, as can all thermoplastics, usingextrusion, injection moulding, inflation techniques and welding.

In a preferred embodiment of the method the first polyethylene componentand/or the second polyethylene component is mixed with the reactionchemicals at ambient temperature in a mixing apparatus prior to theintroduction into the extruder and the bulk material, water-blown by thereaction chemicals, is subsequently introduced to the reaction extruder.This type of processing procedure results in a very homogeneous rawmaterial which, if appropriate, can also be processed in a single-wormextruder into polymer material in accordance with the invention.

In further embodiments of the invention the water cross-linking of themedical instruments produced from the polymer material in accordancewith the invention is carried out at a temperature level which liesbelow the softening temperature of the original polyethylene materialsuntil the shape of the instrument is stabilized by the silanecross-linking reactions. After this forming or shape-fixing, thetemperature of the cross-linking bath can be increased until completecross-linking occurs. In this fashion the time needed for completecross-linking of the polymer material is reduced.

In a further embodiment of the invention the medical instrument isproduced from a silane-grafted polymer material which is cross-linked ina shaping tool.

In a preferred embodiment thereof, the silane-grafted polymer materialis also pressed into moulded extrusions, such as, for example, tubes.The moulded extrusion is subsequently introduced to a shaping tool andthe tool containing the moulded extrusion is brought into a water bath.

The ultra-low or very-low-density polyethylene (ULDPE), VLDPE) arenormally pressed into moulded extrusions in a single step process whichincludes the silane-grafting and shaping. This type of mouldedextrusions (intermediate product), which have a gel-content <5% at theoutput of the nozzle, are subsequently introduced to a shaping tool. Inthe event that a medical instrument is to be manufactured from theintermediate product, a processing of the intermediate product into thedesired instrument must take place prior to the shaping andcross-linking step. Subsequent thereto the shaping tool having theintermediate product or the completed instrument is introduced into awater bath. Cross-linking reactions form a three-dimensionalcross-linked structure from the partial product (extrusion) independence on the susceptibility to grafting, water temperature, andcross-linking time. The cross-linking of the macromolecules takes placeby means of Si--0--Si bridges. The formed cross-linkage fixes thegeometry of the extrusion which pervades the intermediate product duringthe cross-linking phase.

When, after completion of the cross-linking reaction, the formedcomponent is removed from the forming tool, the geometry of the formedcomponent is maintained. This geometry is rigidly formed through themacromolecular cross-linkage and is maintained even under the influenceof steam sterilization. This is a substantial advantage relative tomedical instruments made from PVC.

The polymer material in accordance with the invention and a method forproduction of this material are described and explained more closelybelow in connection with the embodiments represented in the drawings.The features which can be extracted from the description and the drawingcan be used in other embodiments of the invention individually orcollectively in arbitrary combination.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a flow diagram for the production of the polymer materialin accordance with the invention;

FIG. 2 shows a further alternative flow diagram for facilitating themanufacture of the polymer material in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A first polyethylene component 1 or a second polyethylene component 2either alone or in mixture with an organic silane 3 and a peroxide 4 aswell as, if appropriate, a catalyst 5, are thoroughly mixed andsubjected to an increased temperature in the range between 140°≦T≦200°C. This type of heat treatment leads to the formation of silane-graftedpolymer material 6, which can be further processed in the manner ofconventional thermoplastics under dry conditions. For example, this heattreatment can occur in an extruder 7. The first polyethylene component 1or the second polyethylene component 2 or a mixture of polyethylenecomponents travel through the extruder 7 in the direction 7' of materialflow. A vacuum 8 is applied in the end region of the extruder 7 toeffect degasing of the melt. The first and/or second polyethylenecomponents 1, 2 flow into the extruder 7 in a continuous fashion at theproduct input 9. The input product passes, prior to entrance into theextruder 7, through a dosing unit 10, for example a differentialdosaging scale, and is admeasured by weight. The silane-grafted polymermaterial 6 leaves the extruder 7 via the product discharge 9' and issubsequently granulated according to a conventional method. The extruder7 has a heater 11 for heat-treating the polyethylene granulate and thereaction mixture.

The first and/or second polyethylene components 1, 2 having very lowdensity (for example VLDPE, ULDPE) can be produced with specialpolymerization procedures, for example, by "constraint geometry catalysttechnology". These polyethylene components having, preferentially, anarrow molar mass distribution (1.5≦M_(w) /M_(n) ≦3.0, preferentiallyapproximately 2.0) are, for example, silane-grafted in a double-wormextruder through the addition of an organic silane/peroxide mixture. Auni-directionally rotating, densely-combing extruder 7 (double-wormextruder) is particularly well suited for this purpose, since it hashigh mixing and homogenizing efficiency.

In accordance with FIG. 1, the dosing unit 10 transports either thefirst polyethylene component 1 or the second polyethylene component 2or, for example, a mixture of the above polyethylene components ingranulated form into the extruder 7. After the first polyethylenecomponent 1 has been melted, a mixture comprising organic silane 3 andperoxide 4 and, if appropriate, also a catalyst 5 is injected downstreaminto the extruder 7 by means of a membrane dosaging pump and a cooledinjection valve. At this location the reaction fluid is thoroughly mixedwith the first polyethylene component 1 at a reduced temperature (90°C.≦T_(M) ≦130° C.) prior to reactive decomposition of the peroxide 4 dueto further temperature increase along the extruder 7. The organic silanemolecules become coupled to radicals produced in the polymer chain orpolymer chains by the decay of the organic peroxide 4. Appropriateorganic peroxides are, for example, dimethylhexanebutylperoxide (DHBP),dicumylperoxide (DCUP), dibenzoylperoxide (DB) or other peroxide typeswith which decomposition occurs above the melting temperature of thefirst polyethylene component 1. Organic silanes 3 which are particularlywell suited are those with which the Si-atoms are joined withalkoxy-groups. Vinyltrimethoxysilane (VTMOS) and vinyltriethoxysilane(VTEOS) are preferentially utilized here. In addition, catalysts 5(cross-linking catalysts) such as dibutyltindilaurate (DBTL) ortitanylacetonate as well as additional catalysts which enhance thehydrolysis and condensation reaction of the organic silanes can be addedto the reaction mixture comprising peroxide 4 and organic silane 3. Asilane-grafted polymer material 6 having an output gel-content of 0% isavailable after degasing of the melt from which medical instruments canbe produced using conventional methods of thermoplastic processing. Theprocessing of the silane-grafted polymer material can be carried outdirectly or subsequent to a cooling and granulating step.

Since, in contrast to conventional methods for rubber processing, silanecross-linking systems are not formed by a temperature increase ratherthrough the exposure of the medical instrument to a moist environment orthrough storage in water, the storage conditions determine the point intime at which the cross-linking begins and thereby the amount of timeavailable for processing and packaging of the silane-grafted polymermaterial. The cross-linking reactions can also be accelerated by storingthe components made from the polymer material 6 in water at elevatedtemperatures. The water bath temperature must however not exceed thesoftening temperature or the crystallite melt temperature of the polymermaterial (60°≦T_(e) ≦72° C.). Only after the cross-linking reactionshave fixed the shape of the medical instrument, can the water bathtemperature exceed the softening temperature of the raw material(untreated material) to shorten the time for complete cross-linking ofthe polymer material.

In addition to the receptivity, i.e. the organic silane, peroxide, andcatalyst components, the quality of the silane-grafted polymer materialis substantially determined by the homogeneity of the mixing of thesereaction chemicals in the polymer melt. An inhomogeneous distribution ofthe reaction chemicals can lead to quality-reducing pinholes and gelparticles in the polymer material.

FIG. 2 shows a mixer 12, preferentially operating in batch-mode, intowhich the first polyethylene component 1 or alternatively a secondpolyethylene component 2, an organic silane 3, a peroxide 4, and, ifappropriate, a catalyst 5, i.e. a liquid grafting receptor, areintroduced. Both, the first polyethylene component 1 as well as thesecond polyethylene component 2 can be prewarmed. These components arehomogeneously mixed together in the mixer 12 until the fluid hascompletely diffused into the first polyethylene granular component 1. Inthis manner, an exceptionally good predistribution of the graftingreceptors results within the polyethylene matrix. Local cross-linkedclusters and pinholes are avoided. The improved predistribution allowsfor equal gel-content with a reduced amount of grafting receptor.Subsequent thereto, the bulk material mixture 3, water-blown withreaction chemicals, is admeasured into the extruder 7, where the silanegrafting reaction occurs. The homogeneous premixing allows for, in themethod variation represented in FIG. 2, in addition to the conventionaldouble-worm extruder also the utilization of a more economicalsingle-worm extruder having mixing elements. The material flow isindicated by 7' in FIG. 2 and a degasing of the melt is effected by theapplied vacuum 8. The silane-grafted polymer material 6 leaves theextruder 7 at the end region.

Both method variations shown in FIGS. 1 and 2 are operated continuously.

EXAMPLES

The silane-grafted polymer materials I through IV whose composition isgiven in table 1 were produced according to the production variationsindicated in FIG. 1 or FIG. 2.

    ______________________________________                                                   Polymer                                                                              Polymer  Polymer  Polymer                                              material                                                                             material material material                                             I      II       III      IV                                        ______________________________________                                        Polyethlene  100      100      100    100                                     componente "Exact"                                                            Manufacturer:                                                                 Exxon Chemicals                                                               VTEOS (Organic silane)                                                                     2,5      --       --     --                                      VTMOS (Organic silane)                                                                     --       2,5      2,5    2,5                                     DHBP (Peroxide)                                                                            0,08     0,08     0,08    0,08                                   DBTL (Catalyst)                0,02                                           Titanylacetonate                      0,1                                     (Catalyst)                                                                    ______________________________________                                    

After the ULDPE polyethylene component is admeasured into the extrudervia a funnel and melted, the mixture of liquid reaction chemicals(organic silane, peroxide, and, if appropriate, catalyst) is pumped intothe extruder via a diaphragm dosing pump. The silane-grafting reactionoccurs therein. The polymer material leaving the extruder is cooled,and, for example, granulated. The polymer granulate produced from thepolymer material is subsequently processed under dry conditions by meansof an additional extruder into, for example, tubes. A comparison betweenmaterials I and II has shown that the cross-linking reaction occurssignificantly faster when utilizing VTMOS compared to VTEOS. In theevent that the silane-grafted polymer materials I and II are welded insuitable plastic bags in a moisture-tight fashion, both materials remainprocessable even after several weeks of storage. In damp environments orin water, the hydrolysis and condensation reactions occur significantlyfaster with VTMOS than with VTEOS. In addition, the cross-linkingreaction can be increased through the addition of a "master batch" in atube extrusion process. The "master batch" contains either the catalystDBTL or titanylacetonate. In the event that the catalyst is alreadypresent in the polymer material due to the preparation procedure, theavailable packaging time, for example that for the welding ofcomponents, is reduced as is the cross-linking time in the water bath orin the moist atmosphere.

The cross-linked tubes made from materials I through III are astransparent as tubes made from non-cross-linked ULDPE. The cross-linkedtubes made from materials I through III can be easily steam-sterilizedat a temperature of T=134° C. The resistance to buckling of the tubesmade from materials I through III is likewise substantially improvedrelative to the non-cross-linked raw materials. In experiment IV(polymer material IV) a bulk material mixture is produced prior to theextrusion process and the polyethylene components are mixed with theliquid reaction chemicals until same completely diffuse into thepolyethylene components. The bulk material mixture was subsequentlyadmeasured into the double-worm extruder. The polymer material IV is asilane-grafted material having few pinholes and gel particles (highproduct quality). The polymer material IV exhibits a transparencycomparable to that of polymer materials I through III. Polymer materialIV can be steam-sterilized and is non-buckling. In this manner, thepolymer material IV exhibits the same positive product characteristicsas the polymer materials I through III.

A polymer material 6 is formed from a first polyethylene component 1having a low density (VLDPE) which is silane-grafted with an organicsilane 3 in combination with organic peroxide 4 and is cross-linkedthrough storage in a damp environment and/or in water. The polymermaterial 6 can be processed into medical instruments under dryconditions prior to cross-linkage. After cross-linkage the medicalinstruments made from the polymer material 6 exhibit a high degree oftransparency, are non-buckling and flexible. The polymer material 6 isproduced in an extruder, preferentially in a double-worm extruder.

We claim:
 1. A medical instrument comprising:a shaft section having alumen and formed from a flexible polymer, said flexible polymercomprising at least one of a very low density polyethylene component(VLDPE) and an ultra low density polyethylene component (ULDPE), saidflexible polymer being silane-grafted with an organic silane and organicperoxide and being cross-linked by exposure to a water environment. 2.The instrument of claim 1, wherein said flexible polymer comprise 0.5 to5% by weight of organic silane and 0.02 to 0.3% by weight of organicperoxide.
 3. The instrument of claim 1, wherein said flexible polymerhas a gel-content C_(G) of 65%≦C_(G) ≦95%.
 4. The instrument of claim 1,wherein said flexible polymer is cross-linked in a molding tool or unit.5. The instrument of claim 1, wherein said very and said ultra lowcomponents have a narrow molar mass distribution.
 6. The instrument ofclaim 5, wherein said mass distribution is in the range 1.5≦M_(w) /M_(n)≦3.0, M_(w) being a weight-averaged and M_(n) a number-averagedmolecular weight of said components.
 7. The instrument of claim 1,wherein said organic peroxide has a decomposition temperature above amelting temperature of the very low and ultra low components.
 8. Theinstrument of claim 7, wherein said organic peroxide comprises at leastone of dicumylperoxide (DCUP), dibenzoylperoxide (DB) anddimethylhexanebutylperoxide (DHBP).
 9. The instrument of claim 1,wherein a catalyst is added to the flexible polymer.
 10. The instrumentof claim 9, wherein said catalyst comprises at least one ofdibutyltindilaurate (DBTL) and titanylacetonate.
 11. The instrument ofclaim 10, wherein said dibutyltindilaurate is in a range less than 0.05%by weight and said titanylacetonate is in a range less than 0.5% byweight.
 12. Method for producing a medical instrument, the instrumenthaving a shaft section with a lumen therein, the shaft section formedfrom a flexible polymer, the method comprising the steps of:dosing byweight at least one of a first and a second polyethylene component;introducing said first and second components as pourable bulk materialto an extruder; mixing said bulk material with organic silane andorganic peroxide; applying vacuum to said extruder in a productdischarge region to devolatilize a melt; heating said extruder from aregion of input of said organic silane and said organic peroxide to saidproduct discharge region; producing medical instruments fromdevolatilized silane-grafted polymer; and cross-linking said medicalinstruments through introduction of moisture.
 13. The method of claim12, wherein said extruder is a twin screw extruder and said mixing stepcomprises injection of a catalyst, said silane, and said peroxidedownstream into said extruder using a diaphragm dosing pump having acooled injection valve.
 14. The method of claim 12, wherein said mixingstep comprises mixing at least one of said first and said secondpolyethylene components with said organic silane and said organicperoxide at ambient temperature in a closed mixer prior to introductionto said extruder.
 15. The method of claim 12, wherein said cross-linkingstep is carried out at a first temperature below a softening temperatureof said first and said second component to stabilize a shape of themedical instrument and further comprising the step of increasing saidfirst temperature after said shape is stabilized.
 16. The method ofclaim 12, wherein said producing step comprises extrusion ofsilane-grafted polymer into an extrudate, and introducing this extrudateinto a shaping tool, and said cross-linking step comprises introducingsaid shaping tool containing said extrudate into a water bath.